culture result only indicates the presence of the organism, not the toxin production.
Therefore, the test should be used as a relatively sensitive screening test to detect
GDH-positive stool samples that require further testing with tissue culture cytotoxin
assay or EIA. Occasionally, other organisms produce GDH, which lowers the
specificity. The test is rapid, turnaround time 15 to 45 minutes, and relatively
inexpensive.Assays Detecting Toxin
- Tissue culture cytotoxin assaywas the first test described. It has the highest sensitivity
of all the tests and can detect as little as 10 pg of toxin B (26). The assay reveals
cytopathic effects on cell culture monolayers characterized by rounding of fibroblasts
(Fig. 2). Preincubation with neutralizing antibodies against the toxins demonstrates
the specificity of the cytotoxicity. Sensitivity and specificity are high (94–100% and
99%, respectively) (34). It is considered by many experts to be the “gold standard” for
demonstratingC. difficiletoxin the stool. The major disadvantage of the cytotoxin
assay is that it is technically demanding and expensive, and many laboratories lack
the expertise and equipment to provide rapid turnaround (25). - EIAallows direct detection ofC. difficiletoxin (15). Commercially available tests can
detect toxin A only or both toxin A and B. EIA detecting both toxins is preferred since
C. difficilestrains with toxin B only would otherwise be missed. Although rare
C. difficile strains producing only toxin B have caused hospital-based cases.
Advantages of the EIA include fast turn around time (2 hours), relatively easy to
perform, and high specificity (up to 99%). The disadvantage is the low sensitivity
(70–80%) linked to the fact that it requires a large amount of toxins (100–1000 pg) for
detection. The relatively high false-negative rate can be decreased by 5% to 10% by
repeating two to three specimens but this also increases the cost.
Figure 2 Tissue culture cytotoxin assay forClostridium difficile.(A) Normal primary human amnion cells.
(B) Typical changes after application ofC. difficiletoxin. (C) Tissue culture after neutralization withClostridium
sordelliiantitoxin.
Clostridium difficileInfection in Critical Care 279